When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (T.sub.g) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, elastic modulus, tangent delta (tan .delta.), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires. Traditionally, improved dispersion of reinforcing fillers has been accomplished by lengthened mixing times. However, in commercial applications, prolonged mixing times result in decreased production and increased expense.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration (flocculation) of silica particles occurs after compounding, leading to poor silica dispersion and a high compound viscosity. Therefore, it is desirable to improve the dispersion of silica in rubber compounds, especially when used for tire treads, to improve performance characteristics.
Previous attempts at preparing readily processable, vulcanizable silica-filled rubber stocks containing natural rubber or diene polymer and copolymer elastomers have focused on the use, during compounding, of bifunctional silica coupling agents having a moiety (e.g., an alkoxysilane group) reactive with the silica surface, and a moiety (e.g., a mercapto, amino, vinyl, epoxy or sulfur group) that binds to the elastomer. Well known examples of such silica coupling agents are mercaptosilanes and bis-trialkoxysilylorgano polysulfides, such as bis-(3-triethoxysilylpropyl) tetrasulfide which is sold commercially as Si69 by Degussa. With the coupling agent acting as an intermediary, the compound viscosity is reduced and the silica particles are more easily dispersed into the elastomeric matrix. However, such bifunctional silica coupling agents are expensive. In addition, the reaction of the alkoxy portion of the coupling agent with the silica can result in the release of a substantial amount of alcohol, resulting in a rubber compound containing undesirable bubbles that can form blisters or surface defects in the resulting formed rubber articles.
To address the expense and other problems related to bifunctional silica coupling agents, recent approaches to providing improved dispersion of silica in rubber compounds have been directed to reducing or replacing the use of such silica coupling agents by employing dispersing agents, such as monofunctional silica processing aids (e.g. silica hydrophobating agents that chemically react with the surface silanol groups on the silica particles but are not reactive with the elastomer) and agents which physically shield the silanol groups to prevent reagglomeration of the silica particles after compounding. For example, dispersing agents, such as alkyl alkoxysilanes, glycols (e.g., diethylene glycol or polyethylene glycol), fatty acid esters of hydrogenated and non-hydrogenated C.sub.5 and C.sub.6 sugars (e.g., sorbitan oleates, and the like), polyoxyethylene derivatives of the fatty acid esters, and fillers such as mica, talc, urea, clay, sodium sulfate, and the like, are the subjects of co-owned EP 0890606 and EP 0890603. Such silica dispersing agents can be used to replace all or part of expensive bifunctional silica coupling agents, while improving the processability of silica-filled rubber compounds by reducing the compound viscosity, increasing the scorch time, and reducing silica reagglomeration. The use of such dispersing aids includes employing an increased amount of sulfur, to replace sulfur that otherwise would have been supplied by a sulfur-containing silica coupling agent, in order to achieve a satisfactory cure of the rubber compound.
Another approach to improving dispersion of silica filler, involves modification of polymer chains with functional end groups that interact with or shield the surface hydroxyl groups on the silica filler. In particular, alkoxysilane terminated polymers have a carbon-silicon bond and at least one terminal --SiOR group that reacts with the silica surface, forming an Si--O--Si linkage. Although alkoxysilane terminated polymers have provided adequate dispersion of reinforcing fillers during compounding, there has been a problem with stabilizing the Mooney viscosity of the gum polymer prior to compounding. In particular, polymers produced by solution polymerization in inert organic solvents, such as hexane, require a desolventization step after polymerization. Although desolventization can be achieved by drum-drying, in commercial practice desolventization is achieved through the use of either steam or heated water. When the polymer chains are terminated by alkoxysilane compounds, hydrolysis of pendant --SiOR end groups during the desolventization step, leads to coupling of the polymer chains via formation of --Si--O--Si-- bonds between two or more alkoxysilane end groups, resulting in a large increase in the polymer molecular weight and, concomitantly, a large increase in the Mooney viscosity of the gum polymer. Moreover, during storage of alkoxysilane terminated polymers for a period of time prior to compounding, humid environmental conditions and residual water from desolventization can lead to further hydrolysis of end groups and polymer chain coupling, and a further increase in viscosity.
Several approaches have been taken to overcome this hydrolysis and coupling problem. For example, acids such as C.sub.1 to C.sub.12 aliphatic and C.sub.6 to C.sub.12 cycloaliphatic and aromatic carboxylic acids have been employed as viscosity stabilizing agents to treat the polymer prior to desolventization. The acids do not react with the alkoxysilane terminal end groups of the polymer, but rather neutralize the by-product lithium compounds in admixture with the polymer, thereby slowing the rate of formation of low boiling alcohols during desolventization, slowing the rate of the hydrolysis reaction and, therefore, slowing the rate of coupling of the polymer.
Other approaches to controlling the Mooney viscosity of such alkoxysilane-terminated polymers have included the use of alkyl alkoxysilanes, as viscosity stabilizing agents prior to desolventization of the polymer. These agents react with the alkoxysilane terminated polymers by adding a long chain alkyl group to the alkoxy terminal and eliminate, rather than just slow down, any increase in the Mooney viscosity for a period of time.
Another approach to stabilizing the Mooney viscosity of alkoxysilane terminated polymers, involves exchanging the alcohol of the alkoxysilane terminal group with a long chain alcohol, such as an aliphatic, cycloaliphatic, or aromatic alcohol, or a fatty acid ester of a hydrogenated or non-hydrogenated C.sub.5 or C.sub.6 sugar. The presence of the long chain alcohol or fatty acid ester sterically inhibits the availability of the hydrolyzable bond(s) to moisture. This approach results in slowing of the rate of coupling of the polymer which, in turn, slows the foreseen increase in Mooney viscosity of the alkoxysilane terminated polymer.